The rapid growth of electronics, as well as aircraft, automotive and other industries, requires an increasing supply of high-quality metals and alloys, such as tungsten, zirconium, and hard, magnetic and refractory alloys, stainless steels. This trend is accompanied by imposing increasingly stringent requirements on articles of such materials, the chief requirement being that the surface layers of articles should be free from defects, such as microcracks, burns and indentations. In view of the high hardness, viscosity or brittleness of the above-mentioned materials, conventional grinding turns out to be a laborious process which is invariably marked by rapid wear of the abrasive tools. Electrolytic grinding is more effective, but its applications are limited by a number of factors. First, electrolytic solutions tend to corrode the equipment, thereby requiring the critical parts to be made of expensive stainless steel. Second, some metal is dissolved in and reacts with the electrolyte to form hydroxide, which adds to the overall losses of metal; the purification of the electrolyte and the recovery of metal therefrom prove to be laborius processes; and one also has to use big-sized tanks for the electrolyte and complement the equipment with such accessories as centrifuges, settling tanks, etc. Third, electrolytic grinding is a power-consuming process, to say nothing of the difficulties involved in applying heavy current to the tool and the work. Finally, electrolytic grinding requires expensive and cumbersome equipment which is hard to service.
The above disadvantages are eliminated in the grinding process according to USSR Inventor's Certificate No. 494,130 of 1976, Cl. B 23 P 1/10, whereby grinding is carried out with the use of a current-conducting diamond tool in a liquid medium with a conductance of 10.sup.-4 to 2.multidot.10.sup.-2 ohm.sup.-1. cm.sup.-1 and is accompanied by producing electric discharges between the wheel and the work. The discharges serve several purposes. First, they help to avoid the glazing of the tool. Second, they remove the external layer of the current-conducting binder, whereby fresh grains are brought to the wheel's surface. Third, the electrothermal action on the surface of the work is claimed to make the microcutting easier.
An electric discharge is, in fact, a breakdown of the dielectric between the tool and the work, whereafter current reaches both through the freshly formed discharge channel (cf. A. L. Livshitz et al., "Electroimpulsnaya obrabotka metallov" /"Electric Pulse Working of Metals"/, Machinostroyeniye Publishers, Moscow, 1967, p. 27, lines 3-5 from the top).
According to USSR Inventor's Certificate No. 494,130, a pulse generator is used to apply voltage pulses to the electrodes, wherein the tool is the anode and the work is the cathode. The parameters of the spark discharges in the dielectric-filled gap between the electrodes are as follows: frequency, 8 to 440 kHz; on-off time ratio, 1.25 to 5; pulse duration, 1.0 to 100 microseconds; and voltage amplitude, 65 to 200 V.
However, the above method also has its drawbacks. First, it necessitates the use of quite complicated, unreliable in operation, and costly high-frequency pulse generators. Second, the substantial drop of output voltage of such generators at a low load resistance accounts for limited applications of the process.
The latter disadvantage stems from the very principle on which the process under review is based. The high-voltage, high-frequency pulse generators intended to produce spark discharges in the working zone are designed for operation in media possessing a high dielectric strength, such as oil, kerosene, liquid hydrocarbons, etc. On the other hand, such media cannot be used in grinding because of fire hazards. Of course, cutting fluids and 3% soda solution can serve the purpose, but one has to keep in mind that these are aqueous solutions and extremely weak electrolytes. Because of this and due to the presence of chips in the working zone, the resistance of the interelectrode gap is quite low, which accounts for a considerable drop of voltage at the output of the generator. An increase of the area of contact between the tool and the work or an attempt to boost the machine tend to further reduce the voltage level at the generator output and may eventually discontinue the electro-erosion process. According to experiments, the method under review is applicable to tool grinding operations only if the area of contact between the wheel and work is not greater than 2 to 2.5 cm.sup.2 and if the wheel infeed is not in excess of 5 mm/min. It would seem that the problem could be solved by changing construction of pulse generators. However, this would mean using more complicated, bulky and costly generators, wherefore the process would be altogether uneconomical.
In a number of cases, truing is carried out outside the machining zone by means of electro-erosion. This type of truing, however, leads to an erosion and mechanical wear of the truing electrode.